Ok.
1. Go to the folder where you have KSP installed
2. Copy and Paste the KSP folder to your Desktop or any other folder.
3. Delete the KSP folder in your steam directory.
4. Go to Steam -> Right click on Kerbal Space Program -> Properties -> Betas -> select the version you want, in this case 1.2.2
5. Let Steam download KSP and install it.
6. Move the newly installed KSP folder in your Steam Game directory to another location like "Documents" (for example).
7. Move the KSP folder from your desktop back to your Steam Game directory.
8. Go to Steam -> Right click on Kerbal Space Program -> Properties -> Betas -> select "NONE" again.
9. Install your mods in the 1.2.2 KSP install that is now in "Documents" (for example).

I'm a huge IVA fan and I think this is a great mod! The only minor "complain" I have is that you should replace the Apollo Guidance Computer with more "useful" screens like the big RPM ones. I have on savefile where I only play from the IVA and you just need the big monitors to dock for example.

Yo to whoever models the interior, here is a great website where you can check out the Apollo 11 IVA in 3D ! http://3d.si.edu/apollo11cm/
I'd be willing to help with placing props and stuff so just ask me if you need help.